1. The Field of the Invention
The present invention is directed to methods and apparatus for preparing multilayer thin oxide film coatings that have a low net stress. More specifically, the present invention is directed to methods and apparatus for sputter-depositing multilayer thin oxide film coatings which exhibit excellent optical performance, virtually no moisture adsorption, low optical scatter, and a low net stress.
2. The Relevant Technology
Optical filters have numerous diverse applications related to controlling the reflection, transmission, and absorption of light of varying wavelengths. Many filters comprise very thin layers of materials (usually transparent) deposited serially onto the surface of a dielectric or metallic substrate in order to control the way in which the surface reacts to incident light energy. Based on the principle of destructive and constructive interference of light waves, these thin film optical coatings reflect selected portions of the spectrum while transmitting other portions of the spectrum.
The terms "coatings" and "filters" are used interchangeably herein to refer to any type of optical coating deposited on a substrate. Potential substrates include inorganic and organic glasses and similar crystalline materials and metals. Suitable substrates for a particular application are selected on the basis of optical properties, i.e., internal absorption or transmittance, as well as physical and chemical properties affecting the stability of the substrate during exposure to various conditions related to handling and manufacturing of the filter and to the environment in which the filter substrate will be used.
For most optical applications, the coating materials are inorganic, usually consisting of metals, metal oxides (silicon dioxide, titanium dioxide, zirconium dioxide, etc.) and metal nitrides (silicon nitride, aluminum nitride, boron nitride, etc.). Other coating materials include carbides (silicon carbide, germanium carbide, etc.), fluorides, mixtures of metal oxides, or mixtures of oxides and fluorides. The number of layers in the coating may range from a single thin film layer for very simple antireflection or barrier coatings to multilayer stacks of thin films having more than 50 layers for more complex coatings, such as those which separate infrared from visible light.
Depending on the optical application, the physical thickness of the thin film layers can range in order of magnitude from the angstrom range to the micrometer range. A typical thin film layer physical thickness is on the order of 0.1 .mu.m (1000 .ANG. or 100 nm) although relatively much thinner and relatively much thicker layers are commonly used for some types of filters. As used in this application, a "thick" multilayer thin film stack refers to stacks having a physical thickness greater than about 2 .mu.m (2000 nm). In addition to physical thickness, thin film layers may also be usefully described in terms of optical thickness and, particularly, in terms of the quarter wave optical thickness. Thus, it is necessary to specify the type of thickness being described when referring to optical thin films.
As with substrate materials, the coating materials and the physical and optical thicknesses are selected to attain the desired optical properties although the chemical and physical properties of the thin film stacks are a major consideration as well. Composition- and microstructure-dependent properties such as mechanical stress, moisture content, is crystallization and surface morphology of the thin films affect the reliability and performance of the optical device. For example, excessive mechanical stress in an optical coating can result in cracking or delamination of the coating or warping or breakage of the substrate. Moisture content affects the optical performance, e.g., refractive index, as well as the environmental stability of an optical coating. Crystallization can cause stress-induced cracking and rough morphology resulting in optical scattering and loss of mechanical integrity of the coating. Surface morphology also has effects on optical scatter and physical properties of the film.
These properties are affected by factors such as the deposition technique, deposition conditions, deposition rate, material purity and composition, and post-deposition processing such as annealing. Due to the complex interrelatedness of the optical and mechanical properties, both desirable and undesirable effects may occur in response to a particular factor. For example, higher temperatures and lower pressures during coating deposition will typically generate a higher packing density and smaller porosity than low temperature or high pressure conditions. The higher density provides fewer paths for moisture penetration and smaller surface area for water adsorption and, thus, increased moisture stability. High temperatures, however, may not be compatible with some substrate materials, e.g., many plastics, while lower reactive gas pressures do not produce the stoichiometric compositions having the desired optical properties such as low absorbance. Another example is post-deposition annealing which may be used to reduce adsorbed water and increase film density. Annealing, however, may also cause degradation of optical performance due to partial crystallization of amorphous materials, interdiffusion between the layers, structure-related alteration of index of refraction or increase in optical scatter, or thermal stress-induced mechanical failure.
Multilayer thin film stacks comprise at least two different coating materials. For many applications, it is useful to alternate high index (of refraction) materials with low refractive index materials. Silicon dioxide (silica), SiO.sub.2, is a commonly used low refractive index material and is the lowest refractive index material typically deposited with sputter deposition techniques. Thus, multilayer film stacks comprising alternating thin film layers of silica and a high refractive index material are useful for many optical applications.
Evaporation and sputtering are two very useful thin film physical vapor deposition techniques for depositing multilayer thin film stacks. Evaporated thin film layers are typically more porous than sputtered thin film layers. Ion bombardment during deposition with either of these techniques has been shown to advantageously increase the density of the deposited thin films. Silica coatings have an intrinsic compressive stress and the use of silica as the low refractive index material may result in very compressively stressed stacks subject to warping or cracking. Multiple thin layers of silica in a multilayer thin film stack contribute a significant compressive stress, particularly for thick multilayer thin film stacks, i.e., stacks having a physical thickness greater than about 2 .mu.m. The more dense the silica layers, the greater the intra-layer compressive stress. Thus, sputtered silica films, and particularly ion-assisted sputtered silica films, tend to be very highly compressively stressed.
One approach to obtaining a low net stress thin film stack having silica as the low refractive index material is to balance the compressive stress with an identical coating deposited on the opposite surface of the substrate. This approach, however, is not very economical since it requires duplication of a multilayer thin film stack when the desired optical performance can be achieved with a single multilayer thin film stack.
Another approach to obtaining a low net stress thin film stack having silica as the low refractive index material is to balance the compressive stress with a high refractive index material that can provide a compensating, i.e., tensile stress. One source of tensile stress in thin film layers is volume shrinkage that occurs during a post-deposition annealing process. Such shrinkage may be due to crystallization phase changes and/or removal of adsorbed water. Although the crystallization which occurs during annealing results in shrinkage and an increase in tensile stress, the crystallization also may result in increased optical scatter. To minimize this optical scatter, a carefully controlled partial annealing process can be used to transform the microstructure of the thin film layers to an intermediate state between essentially amorphous and significantly crystalline. Because the partial annealing process must be carefully controlled to limit the extent of crystallization, the amount of tensile stress created is also limited. For that reason, the amount of compressive stress that can be balanced is also limited. Because dense films have more compressive stress than porous films, the compressive stress of the silica may also be balanced by depositing porous high refractive index thin film layers. These optical coatings have reduced moisture stability, however, because of the porous thin film layers.
Even though silica has an intrinsic compressive stress, it is possible for a multilayer thin film stack comprising silica alternating with layers of high refractive index material to have an overall net tensile stress due to excessive tensile stress developing within the high refractive index material during the post-deposition annealing process. An excessive tensile stress may also be created in a coating deposited at high temperatures on a low thermal expansion substrate such as fused silica. In addition, depending on the porosity of the thin film layers, the annealing process will remove adsorbed water resulting in shrinkage and an increase in tensile stress in both the silica and the titania or zirconia layers. Thus, even annealing at temperatures below which significant crystallization within the high index material layers occurs, an overall net tensile stress may be created which results in cracking of the film stack or warping and optical distortion in the filter.
One approach to reducing the tensile stress within the high refractive index material utilizes co-deposition of another material, such as silica, to produce composite layers. The composite layers have been shown to have less tensile stress than the pure material layers but the composite layers also have altered optical properties, e.g., decreased index of refraction and increased absorption, which affect the optical performance of the multilayer thin film stack. See, e.g., Russak, M. A., Jahnes, C. V., "Reactive magnetron sputtered zirconium oxide and zirconium silicon oxide thin films," J. Vac. Sci. Technol. A 7 (3), May/June 1989; 1248-1253; Pond, B. J., DeBar, J. I., Carniglia, C. K., Raj, T., "Stress reduction in ion beam sputtered mixed oxide films," Applied Optics, Vol. 28 (14), 1989; 2800-2804; Sankur, H., Gunning, W., "Sorbed water and intrinsic stress in composite TiO.sub.2 --SiO.sub.2 films," J. Appl. Phys. 66 (2), 1989; 807-812.
In view of the above, it will be appreciated that low stress multilayer coatings comprising alternating layers of high refractive index material such as titania or zirconia with the low refractive index material, silica, can be obtained by several known methods. An effective although uneconomical method involves balancing the stress in one coating with an identical coating on the opposite side of the substrate. Another method involves carefully controlling a partial annealing process of evaporated porous thin film layers. Problems with this approach include the difficulty of precisely controlling the annealing process and the moisture instability of the porous layers. Co-deposition of a second material with the high refractive index material has been shown to reduce the intra-layer tensile stress of the high refractive index material, however, at the cost of some reduction in optical performance. Currently, no practical method of co-depositing another material with silica has been found to significantly reduce the compressive stress without adversely affecting optical or environmental durability of the coating.
It will be appreciated that it would be an advance in the art to provide methods and apparatus for preparing multilayer thin oxide film coatings comprising alternating layers of a high refractive index material and silica that have a low net stress and also demonstrate excellent optical performance, virtually no moisture adsorption, and low optical scatter. It would be a further advancement to provide such methods and apparatus which are cost-effective, simple and reliable and which utilize conventional optical coating deposition techniques and equipment.